fix: propagate Shopify 404 through upload route so client retry triggers correctly

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/api/images/upload/route.ts

@@ -26,6 +26,7 @@ export async function POST(req: NextRequest) {
     return NextResponse.json(result)
   } catch (err) {
     const message = err instanceof Error ? err.message : 'Erreur inconnue'
-    return NextResponse.json({ error: message }, { status: 500 })
+    const status = (err as { status?: number }).status === 404 ? 404 : 500
+    return NextResponse.json({ error: message }, { status })
   }
 }